The effects of oxygen flow ratio on the properties of AgxO thin films grown by radio frequency magnetron sputtering

The AgxO thin film with various oxygen flow ratios (R[O2]%) deposited by radio frequency magnetron sputtering (RFM-SPT) has been studied. While adjusting R[O2]% from 0% to 30%, the AgxO thin film transitioned from metal to semiconductor and/or insulator with different transparent appearances on the surface observed using X-ray diffraction (XRD) and transmittance measurement. At high oxygen flow ratios, the AgxO film is multi-phased as a mixture of Ag(II)O and Ag2(III)O3. In addition, the work function (ϕ) of those samples changes from 4.7 eV to 5.6 eV as measured by photoelectron yield spectroscopy (PYS). The compositional and chemical state changes that occur at the AgxO surface during the increments of R[O2]% are evaluated by the relative peak intensities and binding energy shifts in X-ray photoelectron spectroscopy (XPS). With the incorporation of more electrons in chemical bonding, the oxygen-induced band forms. And combining all the results from transmittance (band gap confirmation), PYS (work function confirmation), and XPS (valence band position confirmation), the estimated band diagrams are given for the oxidation state of AgxO with various oxygen flow ratios.


Introduction
Oxide-based thin lm properties are highly benecial in various technological applications including transparent conductive lms, gas sensors, LEDs, transparent electronic devices, and solar cells, among others.][3] According to previous investigations, silver oxide (Ag x O) is a transparent material in wavelengths ranging from the infrared region to the visible region with an optical band gap in the range of 1.2-3.1 eV.Whereas, silver (Ag) is a good reective coating for applications in the optoelectrical eld, 4 Ag x O is preferable for use in battery electrodes as it performs better in voltage regulation and enhances storage life. 5,6The work function can reach roughly 5.3 eV with O 2 plasma or UV-ozone treatment of Ag anodes. 7,80][11] Ag x O has attracted attention as an air electrode in fuel cells and metal-air batteries because of its relatively low Ag-O bonding energy (221 kJ mol −1 ) [12][13][14][15][16][17] enabling it to interact with oxygen reactants and their intermediates without excessively covering the catalyst surface.However, previous studies 7,18 have revealed that the instability of Ag x O resulting from heating is in the transition of its composition, crystallinity, refractive index, and other properties.Furthermore, silver oxides (AgO, Ag 2 O) desorb oxygen at low temperatures (∼200 °C), 18 in consequence, a lowtemperature lm fabrication process is required to form Ag x O thin lms.
2][23][24] In addition, partially oxidized Ag x O lms grown via reactive sputtering were applied for the fabrication of Ag x O/ZnO Schottky diodes (SBDs), 25 Ag x O/a-Ga 2 O 3 metal semiconductor eld effect transistor (MESFET), 26 Ag x O/ IGZO SBDs, 27 and Ag x O/ZnMgO Heterojunction diodes (HJDs). 28Those applications of Ag x O thin lms were proven to improve the barrier height in the interface region of the heterojunction.Indeed, the reactive oxygen might have the capacity to provide O-rich ambient healing of the interface defects or states, for during the sputtering deposition process Ag ion/ plasma would combine with reactive oxygen (oxygen plasma, oxygen radicals).Hence, the purpose of this work is to investigate different oxygen ow ratios of

RSC Advances
PAPER effects on the Ag x O lm properties grown by the radio frequency magnetron sputtering (RFM-SPT) deposition method.

Experimental methods
Ag and Ag x O lms were deposited at room temperature (RT) on quartz substrates which were placed vertically above the Ag target (99.99%purity, 3 inches in diameter) at an appropriate distance of 20 cm by introducing the excited gas mixture of Ar without/with O 2 via a radio frequency magnetron sputtering (RFM-SPT) deposition system.Prior to the lm deposition, the reaction chamber was evacuated to a base pressure below 1 × 10 −4 Pa.During lm deposition, working pressure was maintained at 0.3 Pa.The total ow rate ([O 2 + Ar]) was set at 10 sccm, and the oxygen ow ratio (R[O 2 ]% = [O 2 /(O 2 + Ar)]%) was changed from 0% to 30% while the power supply was xed at 40 W, and the reected power was controlled in the range of 0.5-1 W during the deposition process.Each quartz substrate was ultrasonically precleaned for 2 min in acetone, isopropyl alcohol, and deionized (D.I.) water, respectively, and was then dried using an N 2 gas gun.Before checking the growth rate, the pretest sample preparation was operated for maintaining the chamber ambient and also for removing the top oxidized layer of the Ag target, aer which the thin lm with target thickness was prepared accordingly.Details of the deposition conditions are summarized in Table 1.
The crystal structure was analyzed by grazing incident X-ray diffraction (GIXD) spectra using CuKa radiation (Rigaku Corp., Smart Lab, X-ray wavelength l = 1.5418Å, incident angle 0.35°).When specifying the material, information on each crystal was obtained from the works of literature, [29][30][31][32][33][34] the angle at which the specic X-ray diffraction of Cu was determined, and the measurement results were compared with the theoretical diffraction angle, identifying the material.The thickness of the obtained coatings was measured by a step proler in advance of the growth rate checking, and aer that, the formal deposition with target thickness was operated.The surface morphology was characterized by scanning electron microscopy (SEM) (Hitachi SU8020) with an acceleration voltage of 5 to 15 keV.The optical transmittance of Ag and Ag x O lms was measured in the range from 200 nm to 2500 nm using a UV-visible spectrometer (Hitachi UV-vis U-4100).Aerward, the band gaps were calculated and conrmed by the Tauc plot.The Tauc plot was built on the data extracted from the transformation of transmittance spectra into absorption spectra.The resistances were measured by the resistivity measuring system (MITSUBISHI Chemical Corp., Hiresta-Up MCP-HT450).The work function of 150 nm thick Ag and Ag x O lms on the quartz substrate was determined by photoelectron yield spectroscopy (PYS) (BUNKOUKEIKI Corp., BIP-KV202GD/UVT) in a vacuum at room temperature.Xray photoelectron spectroscopy (XPS) (Ulvac-Phi ESCA 5500 A) was used to investigate the chemical states of Ag and Ag x O lms, and a monochromic Al Ka (1486.6 eV) X-ray source was operated at 96 W and 12 kV at room temperature below 3.0 × 10 −8 Pa.The binding energies in the XPS spectra were calibrated by the peak position of C 1s spectra appearing at 284.8 eV for adventitious carbon surface contamination. 35 3. Without oxygen, the Ag lms of both the 150 nm and 50 nm thickness samples showed good smooth top surfaces.Aer introducing oxygen, the grain shape presented a spherical grain morphology and the grain size became smaller, and the white color particles were characterized by a spherical grain morphology on top of the surface.In the meantime, we can see that the Ag x O lms become much looser and polyporous as the increment of R[O 2 ]% from 20% to 30%, indicating that the surface relocation of silver atoms had increased and the 3-dimensional crystal growth formed at high oxygen ow ratios.

Results and discussion
The variations in transmittance with the wavelength of different R[O 2 ]% of 150 nm Ag x O lms are shown in Fig. 4(a).It can be seen that the transmittances between the R[O 2 ]% of 10% and 16% increased in the longer wavelengths.The absorption edges were narrowly blue-shied between R[O 2 ]% of 10-17%, but aer R[O 2 ]% increased to 20% and 30% the transmittances were decent and the absorption edges were red-shied in a certain range.The relation between the optical absorption coefficient a of a direct band gap (E G ) semiconductor near the band edge and the photon energy hv is given by the following eqn (1): 49,50 ahv = A(hv − E G ) 1/2 (1)    In the certain range of increasing R[O 2 ]%, the ionization energy of Ag x O tended to be high energy shiing of the spectra approaching R[O 2 ]% = 10% obtained approximately 6.2 eV and by increasing R[O 2 ]% to 16% shied to 6.5 eV.However, when R [O 2 ]% is 17%, the yield of photoelectrons was not monotonically increasing with increasing hv and the ionization energy seems to be shied in the opposite direction reaching to 6.4 eV.This may be attributed to the highest resistivity of 17% Ag x O sample with mere electrons so that the 0.    2 and 3.
The C 1s spectra of Ag x O lms with various R[O 2 ]% are shown in Fig. 8(a).0][61] With increments of the R[O 2 ]% from 0% to 30%, the peak positions were shied by 1 eV towards the high energy direction from 287.4 eV to 288.4 eV, in the peaks of surface carbonate their intensities also increased with increasing R [O 2 ]%.From the GIXD data, we determined that with the increments of R[O 2 ]%, the dominant compositions of Ag x O were presumably changed from Ag 2+ to the mixture of randomly distributed Ag 2+ and Ag 3+ .This may be caused by the surface Ag 3+ -adhered carbonate having higher binding energy than the surface Ag 2+ -adhered carbonate.However, for R[O 2 ]% increment of 30%, the extraordinary peak appears as the third peak at 282.2 eV with signicant intensity, which can be expected to be C-Ag bonds from previous studies. 62,63he Ag 3d 5/2 and O 1s spectra of Ag x O with various R[O 2 ]% are shown in Fig. 8(b and c).While the R[O 2 ]% = 0%, the binding energies of Ag thin lm can be observed at 368.4 eV of Ag peak in Ag 3d 5/2 and at 530.5 ± 0.02 eV of Ag x O-related peak in the O 1s spectra.It is postulated that the Ag x O-related peak obtained from the Ag metal sample may be caused by the inuence of the oxidized layer on the surface during atmospheric exposure.As evidence, the intensity of the O 1s peaks of the metallic Ag thin lm is less than that of the corresponding peaks in Ag 3d 5/2 .And there are small peaks present at the  higher binding energy side of both Ag 3d 5/2 and O 1s spectra corresponding to 368.9 eV and 532 eV, respectively.This might indicate the presence of hydroxy groups of AgOH and they are most likely formed by the absorption of H 2 O during exposure to air.However, AgOH is not stable above 228 K, 53,57 so it may be impossible for AgOH to exist in this case but here we only focus on the surface area.The high resolution in the −1-10 eV scan of the valence band (VB) spectra obtained by XPS measurement of Ag x O lms with various R[O 2 ]% is shown in Fig. 9(a).The R[O 2 ]% = 0% VB spectrum presents two peaks at the binding energy of 7.2 eV and 5.5 eV, which may be the result of the Ag 5s and 4d orbital atoms exerting repel forces on each other.This is consistent with the metal-metal interactions for the electric dipole transitions from 5s to 4d orbitals. 65,66Increasing the oxygen ow ratio of R[O 2 ]% from 10% to 17% and 30%, only one peak can be observed with the binding energy changing from 5.8 eV to 4 eV.In the case of R [O 2 ]% = 17% and 30%, the peak wing on the lower binding energy side was lied to a higher intensity compared to R[O 2 ]% = 10% and 16%.This may indicate that with increasing R[O 2 ]%, the atomic interaction preference for metal (Ag) and metal (Ag) transition to metal (Ag) and nonmetal (O) 8 is interpreted as the attractive forces between the charge of Ag 4d orbital and the charge of O 2p orbital bonding to form a hybrid orbital around the valence band. 18,618][69] Meanwhile, Fig. 9(b) of magnifying the valence band region provides good evidence for the interaction of the metal (Ag) and nonmetal (O) bonding system.As R[O 2 ]% = 0%, the density of states at the Fermi level can be properly observed.This is in agreement with the metallic behavior characterized as pure Ag.Aer supplying oxygen R [O 2 ]%, we can observe the oxygen-induced band is forming.In addition to that, these results closely match the ionization energy or work function data from PYS measurements, which shows that high oxygen ow ratios tend to have more oxygen incorporated into the deposited lm, especially the top surface range.Up to now, we can summarize all the information together to dene the main species of Ag x O lms; that is, the samples of Ag  4. Note that the evaluation of the valence band level and the Fermi level difference from the vacuum level is employed using 150 nm Ag x O thin lms, whereas the band gaps were employed using 50 nm Ag x O thin lms because it was hard to obtain the data of band gaps using 150 nm Ag x O thin lms.In polycrystalline materials, the changes in properties with thickness are generally not signicant, 70,71 and this is also conrmed by XRD, SEM, and transmittance measurements for the samples used in this study.For   73 According to this literature, the results obtained in this study can be understood more clearly.In short, a higher proportion of d-band holes and more electrons closer to the Fermi level under the higher valence Ag species were revealed in the literature from the computational studies, and our experimental results showed the same results.

Conclusions
The properties of Ag x O thin lms with various R[O 2 ]% have been investigated and analyzed using different types of measurements.GIXD spectra and SEM images were used for the crystal structure characterization, and UV-vis spectra and the resistance meter were used for the band gap and resistivity investigations.From GIXD data, it was found that Ag x O lms tended to be a mixture of Ag (II)     as a transparent electrode in the infrared light range for heterojunctions and Schottky junctions.The energy level band has been given but the details of the physical phenomena need further conrmations.
Under vacuum ambient, Ag x O lms were deposited on quartz in various oxygen ow ratios (R[O 2 ]%).Photos showing the appearance of Ag x O, which had lengthy exposure to the atmosphere and was put on top of the illuminated screen, are shown in Fig. 1 (a) 150 nm and (b) 50 nm.At the same light illumination, Ag x O samples present different transparency colors, for the increment of R[O 2 ]%.The color-changing tendency is the same for 150 nm and 50 nm lms.More remarkable changes could be seen in (a) 150 nm samples; as Ag x O lm with R[O 2 ]% = 0% presented a silvery metallic color and 10% showed a brown dark color blocking light from passing through.But aer increasing R[O 2 ]% from 16% to 30%, all the Ag x O lms showed lighter brown-grey colors except for R[O 2 ]% = 17% which presented a yellow color.It can be seen that the appearance changes signicantly in relation to the ow ratio of oxygen (the change of x value), and the color-changing tendency follows in 4 categories of oxygen ow ratio: 0%, 10%, below 17%, and above 17%.The inuences of R[O 2 ]% on crystal structure were investigated via GIXD spectra with the thickness of (a) 150 nm and (b) 50 nm as shown in Fig. 2. Focusing on the samples of R[O 2 ]% = 0%, the Ag-related diffraction peaks are detected as: (111), (200), (220), and (311), respectively in both 150 nm and 50 nm

Fig. 1
Fig. 1 Ag x O samples appearance photos comparison with various R [O 2 ]% (a) thickness is 150 nm (b) thickness is 50 nm.

Fig. 3
Fig. 3 Influence of R[O 2 ]% on the surface morphology of Ag x O films (a) 150 nm (b) 50 nm.

Fig. 4
Fig. 4 Variations in (a) transmittance (b) band gaps were extracted from the linear extrapolations of thickness 150 nm samples (c) band gaps and resistivity as a function of R[O 2 ]%.
50 nm Ag x O lms: r = 2.2 × 10 8 U$cm.The resistivity of 50 nm samples is 2 magnitudes higher than that of 150 nm samples.The band gap reached the summit value as R[O 2 ]% = 17% of 150 nm Ag x O lms: E G = 2.5 eV and 1.5 eV, and as R[O 2 ]% = 17% of 50 nm Ag x O lms: E G = 1.7 eV.These results might be interpreted as that aer R[O 2 ]% exceeded 17% provided O ions sufficiency ambient in terms of the formation of the higher chemical state of Ag cations, which in the case of 150 nm depositions of Ag x O formed by sub-stabled Ag 3 O 4 crystals.Aerward, all the measurements and analyses were based on the 150 nm thickness Ag x O sample.The variations in PYS spectra of Ag x O lms depending on the R[O 2 ]% are presented in Fig. 6 (a) full scan range and (b) magnication of small range for determining the work function of Ag and the ionization energy of Ag x O.The monochromatized photon from D 2 (30 W) lamp is used as the excitation light.The density of yield photoelectrons (Y) of Ag x O samples is detected by irradiated D 2 light with incremental photon energy (hv) scan from 4 eV to 9.5 eV, which is thought to be proportional to the square root in the surface area of hv over the threshold ionization energy (I th ) which is equal to the energy difference E 0 − E F for Ag, or E 0 − E v for Ag x O, where E 0 is vacuum level, E F is Fermi level, E v is valence band energy.I th was evaluated through the following equation Y f (hv − I th ) 1/2 (ref.51 and 52) with plotting the hv versus Y 1/2 spectra then by linear extrapolations of Y 1/2 .
1 seconds waiting time yielded a lower density of photoelectrons, and R[O 2 ]% = 16% sample had similar abnormal data.And aer R[O 2 ]% increased to 30%, its ionization energy continued shiing to high energy at 6.8 eV and there was no abnormal behavior of the yield of photoelectron as R[O 2 ]% = 30%.The XPS survey spectrum obtained from the R[O 2 ]% = 17% of Ag x O lm is shown in Fig. 7.The major elements and the minor elements represent peaks in this survey spectrum including the Ag 2p, Ag 3d 3/2 , Ag 3d 5/2 , Ag 4s, and Ag 4p peaks, the O 1s peak and the C 1s peak, and the Ag 5s & Ag 4d peak.The specic information of C 1s, Ag 3d 5/2 and O 1s spectra about the peak assignments and chemical species correspond to different Ag-O compounds with the comparison of the increment of R [O 2 ]% from 0% to 30% obtained by examining the high-

Fig. 5
Fig. 5 Variations in (a) transmittance (b) band gaps were extracted from the linear extrapolations of thickness 50 nm samples (c) band gaps and resistivity with different R[O 2 ]%.

Fig. 6
Fig. 6 PYS spectra of Ag x O films with various R[O 2 ]% (a) full range (b) magnification of small range with linear extrapolation to determine work function.
Meanwhile, as the R[O 2 ]% increased to 10% and 16%, in the Ag 3d 5/2 spectra of Ag peaks intensities were reduced and the proportional increments of Ag x O compound peaks appeared in comparison to R[O 2 ]% = 0%.The binding energies of Ag 3d 5/2 and O 1s were shied to the lower binding energies within the increase in the oxidation state of Ag.Furthermore, we found that by intentionally increasing the R [O 2 ]% from 16% to 17%, even increasing the R[O 2 ]% by just one percent, the splitting peaks of the Ag x O compound shied further to the lower binding energies.From the observation of the Ag 3d 5/2 spectrum of R[O 2 ]% = 17% deposition, AgO dominates the tting fragments of the Ag oxidation state.Whereas in the increase of R[O 2 ]% = 30%, the assignment of the peaks occurred further transitions toward lower binding energies at 366.2 eV of Ag 3d 5/2 spectrum and 528.5 eV of O 1s spectrum, and incorporation of higher oxidation states formed as the mixture of AgO and Ag 2 O 3 .This suggests that at the higher oxygen ow ratio, the oxidation states of Ag are increased with the incorporation of more electrons into the chemical bonds 64 to form the dominant oxidation states of Ag 3 O 4 , which is the mixture of two or more individual oxides of Ag (II) O and Ag 2 (III) O 3 .
x O grown under the R[O 2 ]% = 17% and 30% are the mixture of Ag (II) O and Ag 2 (III) O 3 randomly distributed as Ag 3 O 4 , while those grown under R[O 2 ]% = 10% and 16% are mainly Ag (II) O. Lastly, combining all the causation and correlation, we established a pilot energy level diagram of Ag x O lms with various R[O 2 ]% to illustrate the mechanism between the material character and R[O 2 ]% as shown in Fig. 10.And the comparisons of different R[O 2 ]% inuence on the physical parameters are summarized in Table

Fig. 9
Fig. 9 Measured XPS data for (a) the peaks of the valence-band region and (b) magnifying the valence-band region around E F region of Ag x O films deposited at various R[O 2 ]%.

Fig. 10
Fig. 10 Schematic energy level diagram of Ag x O with various R[O 2 ]%.

Fig. 11
Fig. 11 The transition mechanism of Ag x O with increasing R[O 2 ]%.

Table 1 RFM
-SPT deposition conditions for Ag x O films

Table 2
Binding energies and chemical species of C 1s spectra

Table 3
Binding energies and chemical shifts for different Ag x O compounds O and Ag 2 (III) O 3 .To precisely conrm the band gaps of Ag x O, two different thicknesses of 150 nm and 50 nm were deposited.The PYS spectra were used for work function conrmation.XPS was used to characterize the chemical states of Ag x O.With an increase in R[O 2 ]% from 0% to 30%, Ag x O has a variational orientation for AgO (111) to Ag 3 O 4 (002) grains, and the band gaps have extended to 1.7 eV but narrowed to 1.2 eV.The chemical states of Ag x O have been conrmed as AgO (R[O 2 ]% = 10% and 16%) and Ag 3 O 4 (R[O 2 ]% = 17% and 30%).Therefore, Ag x O has the potential to be used

Table 4
Comparison of the different R[O 2 ]% Ag x O R[O 2 ]% O-state f (eV) E G (eV) E V (eV) jE F − E V j (eV)